1. Field of the Invention
The present invention relates to a spatial light modulator, a spatial light modulator array, an image forming device and a flat panel display which perform light modulation by applying a predetermined voltage between an electrode layer of a support substrate and that of a movable thin film opposed to the support substrate to flexurally displace the movable thin film.
2. Description of the Related Art
FIGS. 15 and 16 show an example of a spatial light modulator in the related art which is produced by a micromachining process including steps of film growth, photolithography, and etching of a sacrifice layer (a later which is to be removed away later in order to form a gap), and in which the light transmittance is changed by an electromechanical operation.
In the spatial light modulator 1, a transparent electrode layer 4 is stacked on an optical waveguide 3 to form a support substrate 6, and a movable thin film 9 which is produced by the micromachining process is opposingly placed above the support substrate with being separated therefrom by a predetermined gap distance. The movable thin film 9 is flexurally displaced by an electrostatic force acting between the support substrate 6 and the movable thin film 9, thereby performing light modulation.
Specifically, the movable thin film 9 has a stacked structure of an electrode layer 11 and an elastic layer 12, and has given transparency. The gap distance between the movable thin film 9 and the support substrate 6 is set by a support 14 which is interposed between the thin film 9 and the support substrate 6.
The movable thin film 9 is enabled to be flexurally deformed toward the support substrate 6 by a gap 16 which is ensured between the thin film and the support substrate 6 by the support 14. As shown in FIG. 15, in a state where a non-driving voltage is applied between the electrode layer 4 on the support substrate 6 and the electrode layer 11 on the movable thin film 9 (for example, the non-application voltage V=0), an attractive force due to an electrostatic force does not act between the electrode layers 4 and 11, and the movable thin film 9 maintains its initial flat state.
In this state where the predetermined gap 16 is held between the movable thin film 9 and the support substrate 6, the device shows optical characteristics in which incident light 18 on the optical waveguide 3 is totally reflected at the surface of the electrode layer 4 and is not emitted toward the movable thin film 9.
As shown in FIG. 16, when a predetermined driving voltage Va is applied between the electrode layers 4 and 11, an attractive force due to an electrostatic force acts between the electrode layers 4 and 11, and the movable thin film 9 is deflected at a predetermined degree toward the support substrate 6 by the electrostatic force, thereby producing a state where the elastic layer 12 is in contact with the electrode layer 4. When this state is caused, the contact interface of the movable thin film 9 does not satisfy the conditions for total reflection of the incident light 18, so that the device shows optical characteristics in which the incident light 18 on the optical waveguide 3 is transmitted through the electrode layer 11 and the elastic layer 12 to be emitted toward the front side of the optical path of the movable thin film 9.
In a spatial light modulator of this kind in the related art, when the driving voltage applied between the electrode layers 4 and 11 in the state of FIG. 16 is canceled, the flexural displacement of the movable thin film 9 is caused to return to the initial state of FIG. 15 by the elastic restoring force of the elastic layer 12, and the incident light 18 cannot be transmitted through the movable thin film 9 (for example, see JP-A-11-258558 and JP-A-2000-214804)
Such a spatial light modulator can be used for a wide variety of applications. Requests for reducing the operating voltage, and increasing the ON/OFF switching speed of a spatial light modulator are growing year by year.
However, a spatial light modulator in the related art in which the returning of the movable thin film 9 to the initial state depends on the elastic restoring force of the elastic layer 12 constituting the movable thin film 9 as described above has a problem in that it is impossible to simultaneously realize the reduction in the operating voltage, and the increase of the ON/OFF switching speed of the spatial light modulator.
When the elastic layer 12 is set to have a weak elastic restoring force, for example, the elastic layer 12 is easily flexurally deformed by a small electrostatic force, so that, even in the case where a low voltage is applied between the electrode layers 4 and 11, the spatial light modulator can operates at a high speed. By contrast, in the case where the elastic restoring force is weak, when the applied voltage is canceled, the returning operation is slow, thereby causing a problem in that high-speed returning cannot be performed.
In order to realize only high-speed returning, it is requested to set the elastic restoring force of the elastic layer 12 to a higher level. When the elastic layer 12 is set to have a strong elastic restoring force, however, a large electrostatic force is required for flexurally deforming the elastic layer 12 toward the support substrate 6. Eventually, high-speed returning must be performed at the sacrifice of the reduction in the driving voltage.